What stations on the current Cardiff to Portsmouth service will not accommodate a 5 car 165/166 formation?
This mainly applies to morning and evening services but I believe Dilton Marsh, Avoncliff, Freshford and possibly Patchway? Would all be too short for a 5 coach Turbo.
Then you also have stations on the Weymouth line when services are strengthened (summer or Xmas markets.)
Fitting a system allowing door selection or local door operation on the Turbos is not just about long formations, it's about making the units more useful, full stop.
An issue on the Cotswold Line ever since the Turbos were introduced has been that the three-car sets cannot call at platforms shorter than they are, due to the lack of any SDO or local door equipment (locking out the doors on one coach while in passenger service is not permitted, before anyone asks), so whenever the booked two-car Turbo is not available to work the Oxfordshire halts services and a three-car stands in, the halts lose their calls.
The issue was eased by the extension of the existing platform and building of a second three-car platform at Ascott-under-Wychwood during the redoubling in 2011 but at Shipton a three-car set can only call when heading towards Worcester, as the Oxford-bound platform is two coaches long. No one will ever pay to extend the platforms at Combe and Finstock, so up until now it has been a two-car or nothing for them.